1. Field of the Invention
The invention relates to techniques for examining the eye, and in particular the back of the eye.
2. Description of the Related Art
The instruments used for this purpose are generally called ophthalmoscopes and serve to perform various different kinds of examination: these begin with simple observation of the retina and extend to more sophisticated applications such as fluorescent angiography, mono and polychromatic examinations, and micro-ophthalmoscopy. For conventional examination of the retina, this type of ophthalmoscopy is performed with a medium field of 20.degree. to 30.degree.. Finally, whatever the application, the level of performance obtained by an ophthalmoscope is essentially defined by its resolution.
The highest performance instruments currently available are retinographs which combine an ophthalmoscope with a camera. These do not provide direct observation, and they are difficult to adapt to dynamic observation of the retina since they are technically limited to taking 3 or 4 photographs per second.
Further, in current ophthalmological practice, ophthalmoscopes are expected to obey Gullstrand's rule.
The application of this rule consists in using different parts of the cornea and the lens of the eye for illumination and for observation respectively. This rule limits resolution and the possibility of observing small details.
A few years ago, experiments were performed to try to associate a vidicon tube with a retinograph in order to display retina structures on a television screen. These experiments did not provide the hoped-for results, in particular because the resolution obtained was small compared with that obtained from a retinographic photograph.
More recently, U.S. Pat. No. 4,213,678 in the names of Pomerantzeff and Coll has proposed a scanning ophthalmoscope. This instrument transposes the flying spot technique used in television, in which a transparent object (eg. a frame of a movie reel) is electronically scanned at television standards. This U.S. patent teaches the use of completely optical scanning using mechanical means to deflect a light beam such as a laser beam in two dimensions through an instrument aperture which is small in comparison with the pupil of the eye. The instrument then collects the optical signal as reflected from any point at full aperture regardless of the incidence, ie. the instrument's field of view is typically 30.degree..
In application of Gullstrand's rule, this prior art ophthalmoscope suffers firstly from the limitation imposed on its resolution by the use of the instrument's small illuminating aperture, and secondly from the use of an observation beam collector which is responsive to the instantaneous beam in the entire field of the instrument. This makes the instrument highly sensitive to reflections and to spurious light, and the above-mentioned U.S. patent seeks to remedy this defect by various optical means, such as the use of polarized light.